1. Field of the Invention
The present invention generally relates to a handoff method and an apparatus using the same and, more particularly relates to a wireless local area network (WLAN) and an apparatus using the same.
2. Background of the Invention
Along with the widespread of wireless local area network (WLAN), voice over WLAN (VoWLAN) is becoming the focus of development. However, current IEEE802.11 standard cannot meet the requirement of VoWLAN due to overlong handoff latencies in WLAN according to existing research data.
WLAN has more handoff opportunities compared to other wireless systems. This is because the coverage of each access point (AP) in a WLAN is not very large (50 m˜100 m), and accordingly the number of handoffs to every VoWLAN user is increased. According to related research, the handoff latency time at the media access control (MAC) layer of a WLAN is between 200 ms and 1000 ms. A mobile station (MS) switch among various working channels to find an appropriate AP when a handoff is started. During this probing period, the MS terminate its connection to the current AP, so that the MS cannot transmit/receive any package. Such continuous package loss in VoWLAN causes conversation pause or termination and accordingly reduces the communication as well as voice quality of VoWLAN.
In conventional handoff mechanism, the handoff latency at the MAC layer of a WLAN is composed of probe delay, authentication delay, and association delay. FIG. 1 illustrates the composition of the handoff latency in a WLAN. Probe delay refers to the delay produced while a MS searches for an appropriate AP in all available channels. The authentication delay refers to the delay produced while the MS performs authentication with the new AP. Association delay refers to the delay produced while the MS re-establishes a network connection with the new AP.
FIG. 2 illustrates the probing process during a handoff procedure at the MAC layer of a WLAN. Channel switch & transmission (CST) refers to the time required by a MS for switching from one to another channel. Min channel time and max channel time respectively refer to the minimum and maximum time required by a MS for probing a channel.
If a MS receives a probe response from an AP on the channel A after the MS sends a probe request to this channel A, the MS wait for the max channel time to obtain all possible probe responses from the channel A. This is because the MS does not know how many APs the channel A has. On the other hand, if the MS does not receive any probe response from the channel A after the MS sends a probe request, the MS waits for the min channel time to probe next channel.
FIG. 3 illustrates a conventional handoff method in a WLAN. When the current signal quality of MS is lower than a predefined threshold value, the MS turns on the handoff mechanism. Here the MS sequentially probes all channels, such as channels 1˜N in FIG. 3. Thereafter, the MS determines which channel has the qualified AP according to a predetermined rule, and the MS switches channels to and re-establishes a network connection with the qualified AP.
According to the method described above, all the channels are probed in sequence before the MS switches channels and re-establishes a network connection. Since the MS cannot communicate with the current AP while probing all the channels, those packages transmitted during the handoff are lost. Accordingly, the conversation may be paused temporarily while the MS probes all channels, which could be very dissatisfying to users of VoWLAN.
In addition, various handoff methods have been further provided. For example, the U.S. Pat. No. 6,278,877 disclosed a patent of “Handover Method for Mobile Wireless Station”. According to this method, a MS passively collects the beacon information issued by APs and determines whether to search for neighboring APs according to the intensities of the beacon signals. The MS determines to handoff to which AP according to the intensities of the beacon signals issued by those neighboring APs. A passive signal detection mechanism is adopted in this patent, namely, the MS statically collects beacon signals issued by neighboring APs.
The U.S. Pat. No. 6,980,535 disclosed a patent of “Handover Method for Mobile Wireless Station”. According to this method, the AP currently connected to the MS issues probe requests to neighboring APs, and the MS statically collects the probe responses sent back by those neighboring APs.
The PCT. WO Patent No. 2004054283 disclosed a patent of “System and Method for Performing a Fast Handoff in a Wireless Local Area Network”. According to this patent, channels are sorted before they are probed so as to reduce the probing time of the MS. The handoff method in this patent is similar to the operation of a neighbor graph, wherein a nearest-neighbor table is stored in each MS and the APs in each nearest-neighbor table are sorted according to historic experiences, and after that, the APs are probed according to their sorted sequence.
Moreover, the U.S. Patent No. 20060092883 disclosed a patent of “Probing method for fast handoff in WLAN”, the U.S. Patent No. 20050282546 disclosed a patent of “Method and system for providing fast handoff in a WLAN-like communication system using active neighboring sets”, the U.S. Patent No. 20060062183 disclosed a patent of “Methods and systems for reducing MAC layer handoff latency in wireless networks”, the U.S. Patent No. 20040224690 disclosed a patent of “Handoff method in wireless LAN, and access point and mobile station performing handoff method”, and the U.S. Patent No. US20060072507 disclosed a patent of “Minimizing handoffs and handoff times in wireless local area networks” etc. These patents are to reduce the time required for probing channels by a MS.